Alosa's right about the bear running up the tree. Escape a bear by climbing a tree? That one's off the list now, along with escaping a bear by already being in a tree. Apparently pointing a camera at the bear and dropping a load in your pants is what works.

When a black bear sow and her cubs were living at East Lake in Oregon, the Mom wasn't shy about wandering through your campground if you where there or not.

A young couple thought it would be cool to sleep at night on the shoreline of the lake "under the stars". During the middle of the night, the girl was awoken by a sniffing sound on her nose.
Sure enough, it was the sow bear. The bear was sniffing the sleeping human. The camper freaked -- rightly so -- and scared the hell out of the black bear and she tore off down the beach with her cubs.